Water is essential for the survival of all known forms of life and 71% of the earth's surface is covered by water, sea water constituting 97% of the total amount of water. Sea water, containing dissolved salts of different chemical compositions, mostly sodium chloride, must be desalinated in order to meet the demand of fresh water from the world. However, there is a large and growing demand for fresh water in many parts of the world. Known technologies for meeting this demand are for example conventional reverse osmosis and centrifugal reverse osmosis.
Conventional reverse osmosis of sea water involves a process wherein the sea water is filtered through a reverse osmosis filter at a high pressure, for example 6-8 MPa. A high pressure feeding pump is required to force the sea water through the filter. Normally, only a part of the sea water is forced through the filter, the major part of the sea water remaining non-filtered. The non-filtered sea water, the concentrate, maintains its high pressure as it is discharged from the device.
Conventional reverse osmosis of sea water has several disadvantages, one of these being the known unreliability of the high pressure feeding pumps as they are often subjected to corrosion and wear problems. However, the principal disadvantage of the conventional reverse osmosis process is the energy being wasted due to the discharge of the high pressure concentrate. Conventional reverse osmosis devices with the extra function of recovering some of this wasted energy are known, however not wide spread due to the additional cost and complexibility.
Centrifugal reverse osmosis is a relatively new technology. In a centrifugal reverse osmosis filtering device the pressure required for forcing the sea water through the reverse osmosis filter is created within a rotating centrifuge body instead of using the above mentioned high pressure feeding pump. The sea water is fed into the center of the rotating centrifuge body at a low pressure and is forced to flow radially outwards and consequently subjected to the influence of the centrifugal force, thereby obtaining a high pressure. A reverse osmosis filter is located in the radially outer region of the centrifuge body and part of the high pressure sea water is forced through the filter. The filtrated fresh water, the filtrate, may be discharged at the periphery of the centrifugal body and the concentrate returns to the center of the centrifugal body to be discharged, thereby regaining its low pressure.
The centrifugal reverse osmosis process does not have the above mentioned disadvantages of a conventional reverse osmosis process. The energy of the high pressure concentrate is not wasted but recovered by the device and no extra function for recovering the waste energy is needed. Furthermore, since no high pressure feeding pump or high pressure pipes are required, the problem of corrosion and wear is less serious. This renders the centrifugal reverse osmosis process a more reliable process for the filtration of sea water into fresh water.
There are several known devices for using the centrifugal reverse osmosis process. For example, WO 98/36823 discloses a centrifugal system for reverse osmosis comprising additional functions for recovering energy also from the filtrate. Kinetic energy from the filtrate is recovered by means of a crown of blades or a spiral shell when discharging the filtrate. Kinetic energy from the filtrate is transferred to the crown of blades and a transmission transforms the kinetic energy into mechanical energy, or the kinetic energy is recovered in the form of pressure by means of the spiral shell which is capable of capturing the rotational flow of the filtrate.